Electrical circuits requiring high power handling capability while operating at high frequencies such as radio frequencies, S-band and X-band have in recent years become more prevalent. Because of the increase in high power, high frequency circuits there has been a corresponding increase in demand for transistors that are capable of reliably operating at radio frequencies and above while still being capable of handling higher power loads.
Metal-semiconductor field effect transistors (MESFETs) have been developed for high frequency applications. The MESFET construction may be preferable for high frequency applications because only majority carriers carry current. The MESFET design may be preferred over MOSFET designs because the reduced gate capacitance permits faster switching times of the gate input. Therefore, although all field-effect transistors utilize only majority carriers to carry current, the Schottky gate structure of the MESFET may make the MESFET more desirable for high frequency applications.
In addition to the type of structure, and perhaps more fundamentally, the characteristics of the semiconductor material from which a transistor is formed also affects the operating parameters. Of the characteristics that affect a transistor's operating parameters, the electron mobility, saturated electron drift velocity, electric breakdown field and thermal conductivity may have the greatest effect on a transistor's high frequency and high power characteristics.
Electron mobility is the measurement of how rapidly an electron is accelerated to its saturated velocity in the presence of an electric field. In the beyond, semiconductor materials which have a high electron mobility were preferred because more current could be developed with a lesser field, resulting in faster response times when a field is applied. Saturated electron drift velocity is the maximum velocity that an electron can obtain in the semiconductor material. Materials with higher saturated electron drift velocities may be preferred for high frequency applications because the higher velocity translates to shorter times from source to drain.
Electric breakdown field is the field strength at which breakdown of the Schottky junction and the current through the gate of the device suddenly increases. A high electric breakdown field material may be preferred for high power, high frequency transistors because larger electric fields generally can be supported by a given dimension of material. Larger electric fields allow for faster transients as the electrons can be accelerated more quickly by larger electric fields than by smaller.
Thermal conductivity is the ability of the semiconductor material to dissipate heat. In typical operations, all transistors generate heat. In turn, high power and high frequency transistors usually generate larger amounts of heat than small signal transistors. As the temperature of the semiconductor material increases, the junction leakage currents generally increase and the current through the field effect transistor generally decreases due to a decrease in carrier mobility with an increase in temperature. Therefore, if the heat is dissipated from the semiconductor, the material will remain at a lower temperature and be capable of carrying larger currents with lower leakage currents.
To provide increased power handling capabilities, transistors with a larger effective area have been developed. However, as the area of a transistor increases, the transistor, typically, becomes less suitable for high frequency operations. One technique for increasing the area of a transistor while still providing for high frequency operations is to use a plurality of transistor cells that are connected in parallel. Such may be provided using a plurality of gate fingers, thus, the source to drain distance may be kept relatively small while still providing for increased power handling capability. When a plurality of parallel transistor cells are connected in parallel on a single chip, the cells are, typically, evenly spaced such that the gate-to-gate distance between adjacent cells (referred to herein as “pitch” or “gate pitch”) is uniform.
When such multi-cell transistors are used in high frequency operations, they may generate a large amount of heat. As a device heats up, performance of the device typically degrades. Such degradation may be seen in gain, linearity and/or reliability. Thus, efforts have been made to keep junction temperatures of the transistors below a peak operating temperature. Typically, heatsinks and/or fans have been used to keep the devices cool so as to ensure proper function and reliability. However, cooling systems may increase size, electrical consumption, costs and/or operating costs of systems employing such transistors.
As discussed above, conventional FETs may be interdigitated structures with multiple unit cells, each unit cell having a source, a drain and a gate. The pitch may determine a temperature rise of the FET. In other words, a wide pitch may be provided to reduce the amount of the temperature rise of the FET. However, FETs having wide pitches may also experience higher drain to source capacitances (Cds), which may not provide desirable device characteristics. In particular, high source to drain capacitances may be undesirable in wide bandwidth amplifiers. Accordingly, further improvements may be made with respect to existing FET devices such that they may provide lower drain to source capacitances (Cds) without sacrificing other performance characteristics of the device, such as thermal device characteristics.